It's really a shame that the iPad, or any tablet or smartphone for that matter, comes with such a limited amount of internal storage -- usually around 128GB at most. And after the operating system and apps, the remaining space for digital content is much less than that. This is the reason a wireless storage expander is a must for savvy users who want to carry their entire digital library around with them.

The good news is there is now a relatively big selection of these type of devices. They are basically portable external drives that come with a Wi-Fi network built in so that you can connect multiple mobile devices to them and stream content stored on them. In many cases, you can even back up user-generated content to one of these, wirelessly.

Following is a list of these devices that I've reviewed in the past couple of years, starting with the oldest of them. This way, you'll see how they're evolving. This list will be updated as more are reviewed.

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Seagate GoFlex Satellite
Released in May 2011, the GoFlex Satellite was the first hard-drive-based wireless storage expander for mobile devices. It's basically a GoFlex portable drive with a 6-hour battery and a Wi-Fi network built in. Up to three Wi-Fi devices can connect to the device wirelessly and stream digital content from it, using a Web browser or the free GoFlex Media app (later renamed Seagate Media), available for both iOS and Android devices. The Satellite has 500GB of storage, whereas the max capacity of the iPad when the Satellite came out was 64GB.

The Satellite can also work as a bus-powered portable drive, just like any other portable drive on the market.

The GoFlex Satellite was revolutionary at the time of its release and worked as expected. Its initial firmware, however, didn't allow it to relay Internet access from another Wi-Fi network. Almost a year later, this feature was added with new firmware released in March 2012. The firmware also increased the maximum number of concurrent Wi-Fi clients to eight (though only three can stream HD content at a time). Read the full review of the Seagate GoFlex Satellite.

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Kingston Wi-Drive
Released just a few months after the Satellite, the Wi-Drive was the first flash-memory-based wireless storage expander. It's very similar to the Satellite in function and features but limited to just 32GB at most. In exchange it's very tiny, even smaller than an iPhone 3GS.

The Wi-Drive also supports a maximum of three Wi-Fi clients at a time but it's capable of relaying Internet access by connecting itself to another Wi-Fi network and allowing the Internet connection to pass through to devices connected to its own Wi-Fi network. When working as a portable drive, the Kingston Wi-Drive only supports USB 2.0, but since its storage capacity is rather limited, this is not a big deal.

Overall, if you just want to moderately expand your tablet's or smartphone's storage capacity and especially want to share hot-spot Internet with multiple devices, the Wi-Drive is a good buy. Read the full review of the Kingston Wi-Drive.

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G-Technology G-Connect
The G-Connect came out in mid-2012 as G-Technology's answer to the Seagate GoFlex Satellite. The company at the time called it an alternative to the then newly introduced iCloud service from Apple.

The G-Connect is very similar to the Satellite, offering 500GB of storage space, but can support five Wi-Fi devices at a time. It also has a network port, making it possible to share a wired Internet connection with mobile devices, which the Satellite can't do. However, it has one major flaw, which is the lack of an internal battery. On top of that, it only supports USB 2.0, making itself out of date right off the bat. Nonetheless, if you stay in a hotel that offers Internet via a network port, it's still quite a decent accessory to have. Read the full review of the G-Technology G-Connect.

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Seagate Wireless Plus
Introduced at CES 2013 and winning Best of CES in the storage category, the Seagate Wireless Plus is the next generation after the Seagate GoFlex Satellite. The device is slightly more compact than its predecessor, yet offers 1TB of storage space, a new battery with about 10 hours of usage time, and a much-improved feature set, including the capability to support up to eight Wi-Fi devices at the same time, and to share hot-spot Internet services that require logging in via a Web page. On top of that, now you can also back up content from mobile devices to its internal storage.

Corsair Voyager Air
The Corsair Voyager Air is the latest mobile storage expander and it basically offers everything discussed above: 1TB of storage space (there's also a 500GB version), built-in Wi-Fi that supports up to five devices with HD streaming, a Gigabit Ethernet network port, and an internal battery that offers about 7 hours of usage.

The device is also very compact, and it supports USB 3.0. It can work as a mobile media server, a home NAS server, or a bus-powered portable drive, and it excelled in all of these roles in my testing. It's not perfect but as far as mobile storage goes, it's the most complete package on the market. Read the full review of the Corsair Voyager Air.

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Patriot Aero Wireless Mobile Drive
The Patriot Aero is somewhat between the Seagate Wireless Plus and the Corsair Voyager Air above. With some 6 hours of battery life, it's similar to the Voyager, and with no network port it's closer to the Wireless Plus. Other than that, it shares what the other two also have in common: it works either as a fast USB 3.0 portable drive or capable, mobile, battery-operated, Wi-Fi-based storage expander. It's only available in 500GB, however. Ready the full review of the Patriot Aero Wireless Mobile Drive.

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SanDisk Connect Wireless Media Drive
The Media Drive is SanDisk's first foray into the mobile storage market and it's quite unique. The device comes with up to 64GB of built-in flash storage but also includes an SD card slot which can host another 128GB (and possibly more in the future). The card slot also means that you can easily use the little device to broadcast photos/video that you have just shot with your digital camera and preview them on your tablet, which has much larger screen than your DSLR. For this reason, the Media Drive is a must-have for photographers. Other than that it has all other features offered by its peers, such as supporting up to five concurrent Wi-Fi devices (running Mac, Windows, Android, iOS, or Kindle Fire), sharing Internet access and so on. It also works as a USB 2.0 portable drive. Read the full review of the SanDisk Connect Wireless Media Drive.

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LaCie Fuel
The Fuel is LaCie's first mobile storage device but it's not entirely new. This is because while it has a totally new design, on the inside, it's pretty much the Wireless Plus from Seagate. The two devices share the same features, storage space, and battery life, and they also use the same Seagate Media mobile app. The reason for this similarity is because the Fuel is the first collaboration between the two company since Seagate acquired LaCie in 2012. That said, the Fuel still offers a great option among mobile storage devices and differentiates itself well from the Wireless Plus with a much more radical design. Read the full review of the LaCie Fuel.